This invention relates to a solid state color imaging apparatus employing a charge transfer device such as a CCD (Charge Coupled Device) and a BBD (Bucket Brigade Device) or a photodiode array, and in particular to such apparatus which senses and stores charges in proportion to an incident light and provides a sequential color video signal.
An area imaging apparatus employing a photodiode array is shown, for instance, in the WESCON Technical papers, session 13-2, Aug. 1967. It consists of photodiodes (or phototransistors), switching elements such as MOS transistors and scanning shift registers. In such apparatus, an electric charge which is proportional to the intensity of the light incident upon each photodiode and is stored therein is read out, when the switching element which is connected to the said photodiode is turned on by being accessed both with a vertical scanning shift register and a horizontal scanning shift register. A sequential video signal representing a black and white image of an object is obtained as an output of this apparatus.
With respect to an area imaging apparatus which operates by means of charge transfer, three methods, the frame transfer method, the inter line transfer method and the horizontal line transfer method have been proposed. An area imaging apparatus employing the frame transfer method is shown, for instance, in IEEE Trans. on Electron Dev. ED - 20, No. 3, p 244, 1973. Such frame transfer imaging apparatus consists of a photosensitive CCD array, a temporary storage CCD array and an output CCD register. A charge image which represents an input optical image and formed by the entirety of the charges stored during one field interval at MIS elements of the photosensitive array is transferred into the temporary storage array during a vertical blanking period. Then it is shifted one horizontal line at a time into the output register during a horizontal blanking period, and read out by a high speed horizontal clock to produce the sequential video signal.
An imaging apparatus employing the inter line transfer method, which is shown, for instance, in 1974 IEEE Intercon Technical Papers, Session 2 -- 2, consists of alternate columns of photosensitive elements, vertical transfer CCD registers and an output CCD register. During a vertical blanking period, signal charges detected by the photosensitive elements are transferred into the vertical transfer registers. These signal charges are then shifted one horizontal line at a time into the output register during a horizontal blanking period, and transferred out by a horizontal clock.
A horizontal line transfer imaging apparatus which is shown, for instance, in RCA Review, vol. 33, September 1972, pp 483 - 500, consists of a parallel array of photosensitive BBD registers, an output BBD register and a vertical scan generator. The signal charges representing one horizontal line signal are transferred by a high speed horizontal clock into the output register and read out therefrom by a high speed clock under the control of the vertical scan generator.
A solid state color imaging apparatus with a single area imaging device may be constructed using any one of the abovementioned solid state imaging apparatuses by additionally employing a striped color filter which is shown in FIG. 1 and a color signal recovering circuit. The striped color filter in FIG. 1 consists of a plurality of sets of striped color filter elements shown as R, B and G which pass the red, blue and green components, respectively, of the optical color image. Such a color filter may be put on the photosensitive area of the imaging apparatus (assume it is a frame transfer imaging apparatus) so that the direction of the repetition of the color filter elements is identical with that of the horizontal charge transfer and each color filter element may cover one photosensitive element in the direction of the horizontal scan or charge transfer. In this apparatus, one horizontal line of signal charges, which represents the red, blue and green signal alternately, is transferred out by the horizontal clock during the horizontal scanning period through the output CCD register. Then, the red, blue and green component signals are obtained from the sequential output of the output register through the sampling circuit which operates synchronously with the high speed horizontal clock and separates them from each other. These color component signals are processed to produce a color video signal.
It is known that the required frequency f.sub.H of the horizontal clock is expressed as; f.sub.H = N/T.sub.H, where N is the number of photosensitive elements which are arranged in the direction of the horizontal scan, and T.sub.H is the horizontal scanning period in seconds. To obtain an adequate resolution for a television camera N should be chosen to be more than 400 and f.sub.H should be 8 MH.sub.Z or higher. Moreover, in case of the abovementioned solid state color imaging apparatus, f.sub.H should be higher than for the monochrome apparatus to secure the same resolution. Such color imaging apparatus requires the output CCD register or the horizontal scanning shift register to be operable under the control of about a 12 - 20 MH.sub.z clock. However, it is extremely difficult to put such a high speed register to practical use.